Fermilab A Plan for Discovery

Chapter 4
The Energy Frontier

The Energy Frontier

Energy frontier colliders are the most powerful microscopes we have. They convert energy into new forms of matter, whose interplay with conventional matter can reveal unknown forces of nature and new physical principles. At the close of the Tevatron era and the opening of the Large Hadron Collider era, scientists have begun to directly examine physics up to multi-TeV energies. No one knows what the experiments at the LHC will find. These experiments aim to discover what generates mass for elementary particles. While both theory and Fermilab data hint at a relatively light Higgs boson, the LHC may reveal a more complex picture with unexpected twists and turns. The ultimate story of the origin of mass may well involve new forces of nature, new energy regimes and a connection to dark matter. Whatever discoveries are made will inspire deeper questions about the organizing principles that give rise to these new phenomena. Discoveries at the LHC will also define the future of physics at the Energy Frontier, guiding the choice of the optimal new accelerator for the next major advance, including a possible Muon Collider at Fermilab.

Fermilab, in partnership with its users, will fully exploit the large datasets collected by the Tevatron experiments. Over the next two decades the laboratory will use its scientific, computing and technical leadership to maximize the discovery potential of the CMS experiment. It will play key roles in planned upgrades to the LHC detectors and accelerator, with significant CMS upgrade activities being carried out on the laboratory site. Fermilab's accelerator and detector R&D programs will create technologies that will enable the next generation of particle colliders.

Theoretical Physics at the Energy Frontier

Fermilab's strong theory group plays a critical role in supporting the national and international high-energy physics program. The group played a central part in conceiving the experimental program for the Tevatron and the Large Hadron Collider and creating theoretical tools for the analysis of collider data, as well as developing ideas being tested experimentally. Members of Fermilab's theory group are closely engaged in today's experiments at the Tevatron and LHC while also assessing the scientific promise of future lepton and hadron colliders.

The Tevatron Collider

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The Tevatron collider

The Tevatron was the world's highest-energy proton-antiproton collider from 1985 until 2011. The Tevatron enabled some of the most important fundamental discoveries of our time, including the existence of the top quark and five baryons, which helped to test and refine the Standard Model of particle physics and shape our understanding of matter, energy, space and time.

The Tevatron collider shut down in September 2011 after 26 years of operation at the Energy Frontier of particle physics. The CDF and DØ experiments will continue analyzing the full data set for several years. Each experiment expects to publish an additional 40 to 50 peer-reviewed journal articles following the end of data taking.

Of particular importance will be the experiments' final results on the search for the Standard-Model Higgs boson. The search should be sensitive at the 95% CL exclusion level or better over the mass range 110–180 GeV and should be complete by the end of 2012. At masses below about 140 GeV, search channels using the bb decay mode contribute significantly to the Tevatron's sensitivity. This is the dominant decay mode at low masses and offers information complementary to the LHC experiments' searches, which rely for now on W+W- or γγ decays in the same mass region.

The top quark was discovered in 1995 by the Tevatron experiments, and the collaborations will complete their measurements of top-quark properties and a precision measurement of the top-quark mass within the next year. Other high-priority analyses that should be complete by the end of 2012 include measurements of diboson cross sections and kinematic distributions, searches for flavor-changing neutral-current decays of heavy-flavor mesons, and measurements of CP asymmetries in B-meson decays. Top-quark and heavy-flavor measurements take advantage of unique capabilities offered by the Tevatron's CP-symmetric initial state. Some additional measurements, such as a precision determination of the W-boson mass using the full data set, are expected to take a few additional years to complete.

The Large Hadron Collider

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The CMS forward pixel detector

Fermilab contributed major components to the CMS detector and is involved in upgrades to the detector and computing systems.

Fermilab is making major contributions to Energy Frontier physics through its strong participation in the CMS experiment at the Large Hadron Collider at CERN, and through its contributions to upgrades of the CMS and ATLAS detectors and the LHC accelerator. With more than 5 fb-1 of data recorded at collision energies of 7 TeV, and with at least two times more data expected by the end of 2012, the CMS and ATLAS experiments are poised to conclusively test the Standard Model and to search for indications of physics beyond it. Fermilab's participation in the CMS experiment includes the activities of its own research group and the critical supporting role the laboratory plays for the collaboration of more than 600 scientists and students from 49 institutions across the United States.

As the host national laboratory for the U.S. CMS collaboration, Fermilab operates a Tier-1 computing center for the CMS experiment, a Remote Operations Center from which U.S. scientists monitor CMS data quality, and hosts the LHC Physics Center. The LPC is a world-leading analysis center for CMS, providing office space, computing resources, software support and U.S. CMS administrative services to an expanding population of users and visitors, as well as hosting numerous schools, seminars and workshops.

The Fermilab CMS group is directly pursuing several research goals, including discovery or exclusion of the Standard-Model Higgs boson for any allowed mass, searches for supersymmetry well above the TeV mass scale, and searches for new dijet resonances or quark compositeness up to several TeV in mass. The group also fulfills essential responsibilities in detector operations and maintenance and provides a large part of the CMS computing services.

The current LHC run plan will conclude 7 TeV collisions at the end of 2012, and then shut down for 18 months for accelerator upgrades, which will allow for 14 TeV collisions in 2014. Taking advantage of Fermilab's experience in detector construction and microelectronics, the laboratory is directly involved in CMS upgrades for operation at higher luminosities and 14 TeV running. Planned activities include instrumenting the outer hadron calorimeter with silicon-based photomultipliers and extensive retooling in software and computing. Fermilab is also heavily involved in CMS upgrades to be deployed in later periods, particularly a new pixel detector, silicon-based photomultiplier instrumentation throughout the hadron calorimeter, and a new silicon tracker.

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The Remote Operations Center

Fermilab plays a critical supporting role for U.S. participation in the CMS experiment at the Large Hadron Collider. Scientists monitor CMS data quality from Fermilab's Remote Operations Center.

In collaboration with the University of Chicago and Argonne National Laboratory, a small group of Fermilab scientists and engineers is involved in the upgrade of the trigger system for the ATLAS experiment.

Fermilab's major contribution to LHC accelerator upgrades will come from its work in high-field magnet technology. The development of high-field superconducting magnets has been central to achieving higher and higher energies in the Tevatron and LHC. Building on the niobium-titanium superconductor technology developed for the Tevatron, Fermilab and collaborating U.S. institutions contributed to the construction of the final-focus magnets for the LHC. This established technology is limited to dipole magnetic fields of 8 to 10 Tesla, however, and plans for an LHC luminosity upgrade in the 2020s require dipole fields of 14 to 16 Tesla. Recognizing this limitation, Fermilab initiated a program to develop magnets based on Nb3Sn superconductor. In the context of the LHC Accelerator Research Program and in collaboration with Brookhaven and Berkeley national laboratories, Fermilab has achieved a major breakthrough in the construction of reliable, accelerator-quality, long Nb3Sn magnets. The technology has advanced beyond the R&D stage and now will be ready for production for the LHC luminosity upgrade.

Computing Support for Tevatron and LHC Experiments

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Dipole magnet for LHC upgrades

Cross section of a demonstrator magnet for an 11-Tesla dipole magnet using Nb3Sn technology. Plans for an LHC luminosity upgrade in the 2020s require dipole fields of 14 to 16 Tesla, while established niobium-titanium technology is limited to 10 Tesla.

The Fermilab Scientific Computing Division stewards the petabytes of data collected by the CDF and DØ experiments and a significant fraction of the multi-petabyte datasets from the CMS experiment. Because the life cycle of these experiments extends over several decades, data preservation to enable a variety of future physics analyses is an important challenge. Data processing and analysis is done using Fermilab facilities in concert with computing centers around the world connected through grid technologies. Fermilab is a founding member of the Open Science Grid and a collaborator on the Worldwide LHC Computing Grid. The distributed computing enabled by these consortia is essential for providing computing for simulations of physics processes, for processing the large distributed datasets, and for making these datasets available to the U.S. and global scientific communities.

R&D for Future Colliders

Over the next decade, experiments at the Large Hadron Collider will explore a new energy regime and uncover the mechanism that distinguishes the weak interactions from electromagnetism. The answer might be the Standard-Model Higgs boson or a more elaborate form of new physics—new forces of nature, new symmetries, new particles or new dimensions of space. Highly sensitive experiments at the Intensity Frontier that aim to detect extremely rare processes will study neutrino oscillations and transitions among different quark and lepton flavors. These experiments will indirectly probe energies beyond those explored directly at the LHC. Discoveries from these experiments will settle some of our most urgent questions, bring others into sharper focus and raise fresh challenges. A diverse and extended experimental campaign beyond the LHC will be needed to address remaining questions and challenges. Future colliders will establish what determines the quark and lepton masses, mixings and degree of CP violation. They may also be needed to tease out the detailed nature of particle dark matter and to give a systematic account of the spectrum, dynamics and symmetries that characterize new phenomena. Fermilab has a strong program in accelerator and detector technology development and fundamental accelerator science for future lepton and hadron colliders, including a superconducting linear collider, a Muon Collider and a high-energy upgrade to the LHC.

The International Linear Collider

To prepare to capitalize on discoveries from the LHC and Intensity Frontier experiments, Fermilab is part of the international effort to develop linear electron colliders, notably the International Linear Collider at 500 GeV. Fermilab has become a world leader in the engineering and technology of superconducting radio-frequency accelerating structures and systems, such as would be required to construct Project X and the ILC. Substantial infrastructure and test facilities have been built at Fermilab to enable development, production and testing of superconducting linear accelerator components and systems. National laboratories, universities, international partners and industry, all coordinating dedicated development activity through the ILC's Global Design EVort, are obtaining accelerating gradients that reach the ILC performance specification with a greater than 50 percent yield, steadily increasing toward the final R&D goal of 90 percent. Through close coordination with the national and international program, they will continue their efforts to achieve ILC beam quality parameters in Fermilab's superconducting test accelerator and will continue to develop and refine processing techniques for achieving a high yield of high-gradient superconducting radio-frequency cavities in a cost-effective manner.

Fermilab and its collaborators work to develop detector systems that are designed to fully exploit the ILC environment to make precision measurements of particle decays. Fermilab has led the development of 3D electronics that can enable the construction of vertex detectors that are 10 times less massive and have position resolution three times better than the current generation. Novel calorimetry techniques are being developed to provide the necessary resolution for hadron showers. An integral part of this work is the design and testing of the low-mass mechanical supports, cooling systems, power-delivery systems and infrastructure essential to the fabrication and operation of linear-collider detectors.

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Components in the MuCool Test Area

An international collaboration of about 200 scientists is working on R&D for a Muon Collider. At Fermilab's MuCool Test Area, scientists test equipment for muon cooling.

Muon Collider

Fermilab scientists are also exploring the feasibility of a multi-TeV Muon Collider, which could be a very attractive complement to the LHC at the Energy Frontier. The Fermilab community is leading physics and detector studies to map out the physics potential of a Muon Collider in terms of the machine's energy and luminosity. Muon Collider detectors would have to withstand large backgrounds from decays of the muon beams. Simulations of this complex environment are currently being performed that will define the detector technologies necessary for the never-before-tested environment of a Muon Collider. Initial studies have shown that tracking detectors and calorimeters with nanosecond timing can reject much of the beam-related background. Once detector performance is understood, the group will begin detailed simulations of the physics to be addressed by a multi-TeV Muon Collider, such as the full spectrum of larger-mass-scale supersymmetry or strong dynamics.

A multi-TeV Muon Collider has many potential accelerator physics advantages over electron colliders, most of which arise from the lack of synchrotron radiation emission by muons, which allows a compact circular design. These advantages include multi-pass acceleration and multi-pass collisions, which could make for a cost-effective approach to reaching high energies with leptons, and a very narrow energy spread. Fermilab leads the national Muon Accelerator Program aimed at developing and demonstrating the concepts and critical technologies required to produce, capture, condition, accelerate and store intense beams of muons. MAP's goal is to deliver results that will permit the high-energy physics community to make an informed choice of the optimal path to a high-energy lepton collider following a focused five-year R&D program. Under study are critical technologies including the MICE experiment's demonstration of transverse muon cooling; RF cavity performance in the presence of high magnetic fields required for muon cooling; and very-high-field solenoids. MAP is also conducting advanced beam dynamics simulations of the muon production, capture, cooling, acceleration and collision processes. The initial application of these new technologies might be in a Neutrino Factory based on a muon storage ring. Project X, under development at Fermilab as part of its Intensity Frontier program, could serve as the front end of a Neutrino Factory or Muon Collider.

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Muon Collider conceptual design

Project X could provide the front end for a possible Neutrino Factory or Muon Collider on the Fermilab site.

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Fermilab's expertise in high-field superconducting magnets will also be critical to a Muon Collider or high-energy LHC upgrade, which both benefit from magnets capable of achieving the highest possible fields. For example, one design for a muon collider requires 50 Tesla focusing solenoids, while a high-energy LHC upgrade would demand 25 to 30 Tesla dipole fields. Such magnets could be based on high-temperature superconductors operating at low temperatures, where they can carry high currents in high magnetic fields. Fermilab is engaged in R&D leading to the construction of the first high-temperature superconductor-based magnets for future Energy Frontier accelerators.